The present invention relates to a laminated polyester film and, more specifically, it relates to a biaxially stretched laminated polyester film having an enhanced adhesion layer formed on at least one surface of a polyester support film, which has excellent adhesive properties. The laminated polyester film according to the present invention has especially excellent adhesive properties to a functional layer containing as a major component a water-soluble or hydrophilic resin such as polyvinyl alcohol or gelatin.
Since polyester films, typically polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) films, are excellent in mechanical strengths, dimensional stability, flatness, heat resistance, chemical resistance, optical characteristics and cost performance, they are used as a support film for various applications. For example, a polyester film is widely utilized as a support for a magnetic tape having a functional layer such as a magnetic layer laminated on at least one of the both surfaces of the support. In recent years, there is an increasing demand for use of an aqueous-type diluent as a diluting solvent for the preparation of a coating composition for the functional layer, because of safety in manufacturing works. To meet with this demand, a water-soluble or hydrophilic resin such as polyvinyl alcohol or gelatin is increasingly used as a binder for various functional layers.
Biaxially stretched polyester films whose surfaces are highly crystalline and which have strong cohesion, have a drawback that the adhesion thereof to a functional layer is weak. To solve this problem, there have been proposed a number of methods such as a method in which a low crystalline polyester layer is laminated on such a polyester film by co-extrusion and a method in which a primer layer such as a water-dispersible polyester resin or an acrylic resin is applied onto such a polyester film (described in Japanese Patent Publication (KOKOKU) No. 49-10243(1974) and Japanese Patent Applications Laid-open (KOKAI) Nos. 52-19786(1977), 52-19787(1977) and 54-43017(1979)).
A polyester film having a primer layer composed of a polyester resin or an acrylic resin, however, has a defect that adhesion properties to a functional layer containing a water-soluble or hydrophilic resin such as polyvinyl alcohol as a major component are unsatisfactory.
As a result of the present inventors' earnest studies to solve the above problem, it has been found that a laminated film stretched in at least one direction, comprising a polyester film support and an adhesive layer having a specific composition, is excellent in adhesive properties to a functional layer containing as a major component a water-soluble or hydrophilic resin. The present invention has been attained on the basis of the above finding.